The present invention is directed to preparing metal nitride and/or metal oxynitride particulate material, such as nanoparticles, microparticles, nanotubes, bulk powder, and to form coatings from organometallic precursors. Additionally, the invention is directed to making organometallic precursors that can be used to prepare metal nitride and/or metal oxynitride particulate material.
Refractory nitrides are useful materials with numerous industrial applications. For example, niobium nitride (NbN) is a hard, metallic refractory, which is silver in color [Ref. 1] and has superconducting properties (Tc=17.3 K) [Ref. 2]. Tantalum nitrides have more widespread commercial usage than niobium nitride. The two most common forms of tantalum nitride are tantalum(V) nitride (Ta3N5) and tantalum(III) nitride (TaN). TaN is a hard refractory, metallic material, which is silver in color. It has good shock and heat resistance leading to its use in corrosion and wear resistant coatings. TaN thin films are suitable for use as diffusion barriers in integrated circuits. Tantalum nitride exists in a large variety of stable and metastable phases. There are three known thermodynamically stable phases, namely γ-Ta2N, Ta5N6, and Ta3N5. The electrical properties of these phases varies from nitrogen deficient γ-Ta2N to nitrogen rich Ta3N5, from metallic γ-Ta2N, ∈-TaN, δ-TaN, and Ta5N6 to completely insulating Ta3N5. Although most of the applications involve conducting-TaNx-materials, two applications of the insulator, Ta3N5, can be considered. First, it can be used as a visible-light driven photoelectrode material for the conversion of photon energy into chemical energy (e.g., the splitting of water). The band positions (conducting band, valence band) of Ta3N5, enable this material to act as a photocatalyst [Ref. 3]. A second application is the use of Ta3N5 as a gate dielectric in the fabrication of MOSFET transistor-structures [Ref. 4]. Based on the mechanical properties, these materials are applied in the formation of thin coatings on tools in order to improve their wear resistance.
Stable phases of vanadium nitride are VN and V2N. VN crystallizes in the NaCl (Fm3m) structure and V2N crystallizes in the hexagonal structure (C632). VN has been tested for application in super capacitors. VN coatings are used in mechanical tools. VN powders show some exceptional catalytic properties due to a vanadium oxide coating formed on the surface. VN is a super conductor with transition temperature of 8.2K.
Among the above-noted metal nitride compositions, only Ta3N5 possesses the required band gap for absorption of light, and effectively acts as a photocatalyst in splitting water. Solid solutions between Ta, Nb, and V nitrides may produce new materials with superior photocatalytic properties.
Various methods have been reported for the synthesis of nanocrystalline metal nitrides in the prior art. Most common methods of metal nitride synthesis involve high temperature reactions between the metal oxide or metal chloride under a constant flow ammonia gas [Ref. 5]. The synthesis of single-crystalline metal nitrides using the metathesis reactions between metal chlorides and NaN3 or NaNH2 have been reported recently [Ref. 6]. The metathesis reaction involving TiCl4 and Ca3N2 also produces TiN nanorods [Ref. 7]. A self-propagating combustion synthesis of NbN has also been reported. Gomathy and Rao teach the use of synthesis of NbN, BN and TiN by reacting urea with a corresponding metal compound [Ref. 8]. Conventionally, VN has been synthesized by various high temperature methods, such as the direct reaction of metal vanadium with nitrogen at 1200° C. [Ref. 9], carbothermal reduction of vanadium pentoxide in N2 at about 1500° C. [Ref. 10], and solid-state metathesis (SSM) routes at elevated temperatures [Ref. 11]. However, most of these reactions involve processing temperatures higher than 1000° C. for extended time periods. Qian et al. recently reported a facile approach to prepare VN at room temperature, using vanadium tetrachloride (VCl4) as the vanadium source and sodium amide (NaNH2) as the nitrogen source [Ref. 12].
Metal nitride films have been generally deposited using chemical vapor deposition (CVD) involving the reaction of metal chloride with ammonia in the presence of hydrogen at temperatures above 900° C. This reaction releases corrosive hydrochloric acid vapor, which along with the high deposition temperatures, makes the process largely unsuitable for microelectronic applications. Deposition of NbN films from Nb(NMe2)4/NH3 has been reported at temperatures from 200 to 450° C. [Ref. 13]. This reaction does, however, produce organic contaminants. Niobium nitride thin films have also been prepared by sputtering niobium metal in a nitrogen atmosphere. TiN by CVD of TiCl4 and hexamethylenedisilazane (HMDS) [Ref. 14].